Correction Apparatus and Method for Imaging Signals and Video Camera
This invention relates to a correction method and apparatus for imaging signals for flare correction and a video camera having the flare correcting finction.
Heretofore, in a video camera in which an image of an object formed on an imaging surface of an image pickup device by the imaging light incident thereon via an image pickup optical system is converted into electrical signals, which electrical signals are outputted as image pickup signals, a phenomenon termed flare is sometimes produced. This flare is a phenomenon in which the incident light is reflected by an image pickup surface or plural lenses of a zoom lens unit to fall on an image pickup device to raise (or float) the signal level of dark portions of the image (the signal level of the entire image) than an actual level. In particular, a lead oxide (PbO) layer forming a photoelectric conducting surface of a photoelectrically conductive image pickup tube used in a conventional video camera, absorbs the red light to a lesser extent and reflects it, this reflected light being reflected by a surface plate glass to be reincident on the PbO layer to raise the signal level of the dark portion or to cause flare responsible for color distortion, all in a well-known manner.
For this reason, it is practiced in the video camera to correct the above flare. The correction level necessary for flare correction, that is the 0 flare correction level, is said to be proportionate to the average value of the incident light volume, that is proportionate to the average picture level APL. Therefore, the flare correction circuit loaded on the video camera integrates picture signals obtained from an image pickup device to detect the APL and subtracts the APL multiplied by a preset coefficient as the flare correction level from the original image to prevent the signal level of the dark portion from being increased.
In a video camera used in, for example, a broadcasting station, camera adjustment is made using a gray scale chart 10 as shown in FIG. 1. This gray scale chart is prepared by bonding a paper sheet with a prescribed reflectance on a 4:3 picture frame. A 11-stage or a 9-stage gray scale chart is commonly used. The gray scale chart 10 shown in FIG. 1 is a 11-stage gray scale and has a reflectance of a white area 10W with the maximum reflectance of 89.9% and has a reflectance of a black area 10B with the maximum reflectance of 2%.
Using the gray scale chart 10, the white portion is matched to 100% of the picture signal level (100 IRE (Institute for Radio Engineers)). A video engineer (VE) of a broadcasting station performs gamma correction, knee point adjustment or flare correction required for video camera setup, before starting the program recording or relaying.
The signal waveform of image signals, obtained on imaging with a routine video camera, and observed by a measurement equipment, termed a waveform monitor, is shown in FIG. 2.
In the image signals of the signal waveform, shown in FIG. 2, the white portion corresponding to the white area 10W, has the maximum signal level. If this signal level is adjusted to 100 IRE, the signal level of the black portion corresponding to the black area 10B is 100xc3x972/89.9=2.2 IRE.
However, in an actual video camera, the vicinity of the black level is amplified by a factor of approximately four by gamma correction. There is also a function termed a pedestal in which the complete black level is not set to 0 IRE but the signal level is raised to prevent collapsing of the black and its vicinity, such that, in the total absence of the incident light, pedestal level of the order of approximately 51 RE is added. There is also produced a phenomenon, termed flare, in which he black level is slightly floated by the flare effect caused by the random scattering of the incident light in the inside of the lens or on the imaging surface. Thus, the black level in the signal waveform of the imaging signals is approximately 2.2xc3x974+5+ flare effect or 15 IRE.
Since the flare inherently ins proportionate to the average value of the incident light volume, that is the average picture level (APL), the APL is detected by integrating the picture signals obtained from the image pickup device and the APL thus detected is multiplied with the flare correction coefficient to give a flare correction level which is then subtracted from the original picture signals to correct the flare.
Specifically, with the signal waveform of the imaging signals, shown in FIG. 2, the black level is of the order of 15 IRE due to the flare effect of approximately 1.2%. In the incident light volume which gives the signal waveform of the image pickup signals shown in FIG. 2, the flare correction level of 1.2 IRE obtained on multiplying the APL of the imaging signal with the flare correction coefficient is subtracted from 15 IRE to give the black level of 13.8 IRE. With a video camera having the flare correction function, the flare correction level is doubled in a manner corresponding to the doubled flare effect, even if the iris is opened by one throttle level to give an excess incident light volume. Thus, the flare effect can be canceled to maintain the black level at 2xc3x97(2.2xc3x974)+5=22.6 IRE.
The structure of a video camera having a conventional flare correction function is shown in FIG. 5.
In a color video camera 30 shown in FIG. 5, the light from an object, incident via an image pickup lens optical system 11 on an image pickup unit 12, is separated by a color separation prism, not shown, in the image pickup unit 2, to give three color beams, that is red (R), green (G) and blue (B) beams, which are incident on associated image pickup devices, not shown. The image pickup devices, associated with R, G and B, convert the incident light beams R, G and B into electrical signals (imaging signals of the respective color components). The imaging signals of the respective color components from the image pickup devices are amplified by amplifiers in the image pickup unit 12 to output signals of a require signal level which are outputted.
The output imaging signals of the image pickup unit 12 are sent to associated variable gain amplifiers 13R, 13G and 13B where the imaging signals are adjusted for white balance so that the white portion of the object will be of the correct white color, that is so that, for imaging signals of the respective colors obtained from the white portion of the object, correct white color picture signals will be obtained on subsequent conversion of the imaging signals into picture signals. The imaging signals outputted from these variable gain amplifiers 13R, 13G and 13B are entered to subtractors 14R, 14G and 14B.
The output signals from the subtractors 14R, 14G and 14B are sent via associated amplifiers 15R, 15G and 15b to an APL detection circuit 16 and to an image enhancer 19.
This APL detection circuit 16 integrates the imaging signals of the respective color components R, G and B over several frames to detect an average signal level APL of the imaging signals associated with the respective color components. The APL signals associated with the respective color componentsas detected by the APL detection circuit 16 are sent to a coefficient multiplication circuit 17.
This coefficient multiplication circuit 17 multiplies the APL signals as found from one color component to another with the flare correction coefficients associated with the respective color components supplied from a system control micro-computer 18. The product values resulting from multiplication by the coefficient multiplication circuit 17 represent the flare correction levels associated with the respective color components. The relation between the APL signals, the flare coefficients multiplied with the APL signals and the flare correction level is shown in FIG. 6, in which the ordinate and the abscissa stand for the flare correction level and the APL values, respectively.
The flare correction signals, representing the flare correction level corresponding to the respective color components obtained by the coefficient multiplication circuit 17, are sent to the subtractors 14R, 14G and 14B fed with the imaging signals from the variable gain amplifiers 13R, 13G and 13B, that is the original imaging signals. The output signals of the subtractors 14R, 14G and 14B are adjusted in level by the associated amplifiers 15R, 15G and 15B and thence supplied to the APC detection circuit 16 and to the image enhancer 19.
This image enhancer 19 is used to enhance the edge for the contour of an image, if need be, to improve the picture quality. The signals of the respective color components, improved in picture quality (enhanced in contour) by this image enhancer 19 are sent to a process circuit 20 which then adds a pedestal level to the picture signals and performs so-called knee correction, gamma correction or white clipping on the picture signals having the pedestal level added thereto to send the resulting signals to a transmission/encoder circuit 21 of the next stage.
A transmitting portion and an encoding portion of the transmission/encoder circuit 21 are used for transmitting signals imaged by the camera to a camera control unit (CCU), not shown, and for processing the imaging signals of the R, G and B components for conformity to different signal standards. The signals from the transmuting portion are sent via a terminal 22 to the camera control unit, while those from the encoder portion are sent as VBS signals, that is as analog composite signals, or as SDI signals, that is serial digital video signals, to a downstream side circuit, not shown.
The system control micro-computer 18 not only generates flare correction coefficients to be sent to the coefficient generating circuit 12 but also controls the image pickup lens optical system 11, image pickup unit 12, image enhancer 19, process circuit 20 and the transmission/encoder circuit 21.
When used in, for example, a broadcasting station, the color video camera 30, described above, is adjusted as follows:
That is, the iris of the image pickup lens optical system 11 is closed and the pedestal level is adjusted to a magnitude prescribed from one broadcasting station to another, as shown in FIG. 4. In general, the three channels of R, G and B are adjusted to the same signal level.
The iris is then opened and the white level of the imaging signals obtained on imaging the gray scale chart shown in FIG. 1 is matched to 100 IRE, as shown in FIG. 2. This sets the black level to approximately 15 IRE.
By flare control of each channel, the black levels of the three channels of R, G and B are matched to the values prescribed from one broadcasting station to another (13 to 17 IRE).
Meanwhile, as an image pickup device provided in the image pickup unit in the above-described color video camera, a solid state imaging device, such as a CCD image sensor, has come to be used in place of a conventional imaging tube.
The CCD image sensor has come to be used as an image pickup device and the image pickup lens has also been improved by reflection inhibiting coating to lower the flare level of the imaging signals themselves.
Notwithstanding, it is a frequent practice used in broadcasting stations to set the black level in the same way as in the video camera employing the conventional imaging tube, in view of interchangeability with the conventional broadcast program resources, even with the video camera having the CCD image sensor as an image pickup device.
That is, if, with the conventional video camera employing the imaging tube, the above-mentioned 11-stage gray scale chart 10 is imaged, the black level is adjusted by flare control so that the signal level of the image pickup signals corresponding to the black level 10B will be 15 to 17 IRE. Similarly, if, with the video camera employing the above-mentioned CCD image sensor, the gray scale chart 10 is imaged, the signal level of the imaging signals corresponding to the black area 10B is adjusted by flare control to be equal to 15 to 17 IRE by flare control.
In the case of the video camera employing the CCD image sensor, the signal level of 15 to 17 IRE controls the coefficients in a direction of reversely floating (raising) the signal level of the dark portion responsive to the APL rather than correcting the flare. That is, if the flare correction coefficient is set to a negative value as indicated by a broken line in the drawing, the signal level of the dark portion is floated to a level more than is necessary for high APL.
That is, if, in a video camera employing the above-mentioned CCD image sensor, the conventional flare correction technique of subtracting the product of the APL with the flare correction coefficient, herein of a negative value, from the original imaging signals, optimum flare correction cannot be achieved. That is, if the incident light volume is relatively low, as when imaging the gray scale, there is raised no particular problem. However, if the APL is of an extremely large value, as when relaying signals outdoors, the flare correction function of preventing floating of the signal level of the dark portion is not in regular operation, but rather operates in a direction of additionally floating the signal level of the dark portion.
It is therefore an object of the present invention to provide a correction method and apparatus capable of performing optimum flare correction even with the use of the CCD image sensor as an image pickup device, and a video camera having the flare correcting finction.
For accomplishing the above object, the present invention provides a correction apparatus for imaging signals including average signal level detection means for detecting average signal levels of imaging signals obtained by an image pickup device, setting means for permitting a user to optionally set a plurality of correction levels associated with a plurality of the average signal levels, correction signal generating means for generating correction signals specifying correction levels, based on the information as set by the setting means, from the average signal level as detected by the average signal level detection means, and subtraction means for subtracting the correction signals from the imaging signals obtained by the image pickup device.
The present invention also provides a correction method for imaging signals including a step of detecting an average signal level of imaging signals obtained by an image pickup device, a step of permitting a user to optionally set a plurality of correction levels associated with a plurality of average signal levels, a step of generating a correction signal specifying a correction level from the detected average signal level based on the as-set information, and a step of subtracting the correction signal from imaging signals obtained by the image pickup device.
The present invention also provides a video camera including image pickup means for imaging an object by an image pickup device, average signal level detection means for detecting the average signal level of imaging signals obtained by the image pickup device, setting means for permitting a user to optionally set a plurality of correction levels associated with a plurality of the average signal levels, correction signal generating means for generating correction signals specifying correction levels, based on the information as set by the setting means, from the average signal level as detected by the average signal level detection means, and subtraction means for subtracting the correction signals from the imaging signals obtained by the image pickup device.